1. Field of the Invention
The present invention pertains to drilling operations, and, more particularly, to the assessment of adverse down-hole drilling conditions.
2. Description of the Related Art
In many types of drilling operations, there is a great deal of interest in the drilling conditions encountered by the drilling equipment in the borehole. The reasons are many, but the interest primarily arises from the fact that even minor interruptions in drilling operations can be quite expensive. Many types of interruptions can be very expensive. Current economic conditions in the industry provide little margin for error with respect to costs. Thus, drilling companies have a strong incentive to avoid interruptions of any kind.
Gathering information about down-hole drilling conditions, however, can be a daunting challenge. The down-hole environment is very harsh, especially in terms of temperature, shock, and vibration. Furthermore, many drilling operations are conducted very deep within the earth, e.g., 20,000′–30,000′, and the length of the drill string causes significant attenuation in the signal carrying the data to the surface. The difficulties of the down-hole environment also greatly hamper making and maintaining electrical connections down-hole, which impairs the ability to obtain large amounts of data down-hole and transmit it to the surface during drilling operations.
Approaches to these problems are few in terms of assessing adverse down-hole drilling conditions. Non-threatening condition may be recorded, displayed, or analyzed by a computing device as well. In general, data taken from the surface and only limited data taken from the surface and/or the bottom of the borehole is available. The drilling operators must extrapolate the down-hole drilling conditions from this data. Because the borehole might be as deep as 20,000′–30,000′, surface data frequently is not particularly helpful in these types of extrapolations. The down-hole data can be more useful than surface data, but its utility is limited by its relatively small amount and the fact that it represents conditions localized at the bottom of the bore. Thus, the down-hole data may be useful in detecting some conditions at the bottom of the borehole but of little use for other conditions at the bottom or along the length of the drill string.
In downhole drilling applications, drilling fluids or drilling muds are circulated through the drill string and annulus of the borehole to remove cuttings from the borehole, lubricate and cool the drill bit, stabilize the borehole, control formation pore pressure, and the like, as a drill bit penetrates the earth. In conventional “overbalanced” drilling, the pressure of drilling fluids circulated through the drill string is typically maintained higher than the downhole formation's pore pressure. This provides a stabilizing function by keeping formation fluids, such as gas or other hydrocarbons, from overcoming the pressure of the drilling fluid, possibly causing a dangerous kick or blowout at the surface.
Although conventional overbalanced drilling has been recognized as the safest method of drilling, it has several drawbacks. Since the drilling fluid pressure is maintained higher than the formation's pore pressure, the formation is easily damaged by the intrusion of drilling fluids into the formation. For example, overbalanced drilling may cause the blockage or washout of the formation structure. In addition, because the drilling fluid pressure exceeds the formation's pore pressure, the penetration speed of the drill bit may actually decrease. This occurs because cuttings produced by the drill bit are often inadequately removed in overbalanced systems, thereby causing the drill bit to rotate against the buildup of cuttings rather than penetrating through virgin rock. This also decreases the life of the drill bit, thereby requiring more frequent drill bit replacement and loss of drilling time.
To overcome some of the disadvantages of “overbalanced” drilling, “underbalanced” drilling has been used and developed. In underbalanced drilling applications, the drilling fluid pressure is maintained below the formation pore pressure. In such applications, a well may actually flow while it is being drilled. Underbalanced drilling provides several significant advantages compared to overbalanced drilling.
For example, because the drilling fluid pressure is less than the formation pressure, the penetration of drilling fluid into the formation is reduced, thereby reducing damage to the well. Since formation damage is reduced, stimulation needed to initiate well production is also lessened. Moreover, drilling penetration rates may increase significantly because the higher formation pore pressure may naturally urge cuttings away from the cutting surface as they are removed by the drill bit. Thus, better contact is provided between the drill bit and virgin rock. Also, since filter caking (i.e. caking around the well bore caused by the penetration of drilling fluids into the formation) is reduced, sticking between the drill sting and the borehole is also reduced. Perhaps even more importantly, the decreased drilling fluid pressure in underbalanced applications can help detect potential sources of hydrocarbons that may go undetected using convention drilling techniques.
Nevertheless, underbalanced drilling also presents certain challenges. First, underbalanced drilling is more subject to blowouts, fires, and explosions caused by the formation pore pressure overwhelming the lower pressure of the drilling fluid. Second, due to the precise control and monitoring needed, underbalanced drilling can be more expensive than conventional drilling. Also, because of the decreased pressure, the removal of cuttings can be problematic, especially in directional drilling applications where the well deviates from vertical or is substantially horizontal.
For instance, one adverse drilling condition of interest is “stuck pipe.” As the drill sting bores through the earth, the borehole seldom descends straight into the earth. There typically are many deviations from the vertical, and some may be very severe in some drilling applications. In these situations, the sides of the borehole may bind the drill string causing it to become stuck within the borehole. Once the drill string becomes stuck, it is quite costly to halt drilling operations and free the drill string.
Currently, stuck pipe is quite easy to detect at the surface once it occurs. Early indications that a stuck pipe condition is developing may be garnered from torque measurements made at the top of the drill string, i.e., at the surface. However, there is value in knowing not only that a stuck pipe condition is developing, but where in the borehole it is occurring. Current techniques cannot provide this kind of information because the data they work from has insufficient granularity.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.